Creating a guild for having a central drop-off/retrieval/storage areas for all your toons = win as a multiboxer. I still need to purchase my last two tabs, but epic mounts for my second team which just turned 80 are first.

In terms of annoying guild invites, a more specific solution is Jafula's Jamba boxer addon. Among its innumerable awesome features, it supports auto-group, guild, and duel declines.